1. Field of Invention
The invention relates generally to improving cables containing optical fibers, and more specifically to apparatuses and methods which are used to increase the mechanical load capacity of the cable and to decrease the optical system noise introduced through the cable.
2. Art Background
Cables are used in various industries to transmit optical signals from point to point over long distances. Optical fibers are a preferred transmission line in many cases due to the low signal attenuation presented by the optical path within the optical fiber. Such a cable can contain one or more optical fibers which provide transmission lines for the optical signals. Existing cable technology utilizes optical fibers, which are typically made from glass; a protective layer that houses the optical fiber, referred to as a “buffer tube;” and one or more layers that provide mechanical strength. The layers that provide mechanical strength are sometimes called “strength members” or the layers can contain “strength members.” The strength members and buffer tube make the resulting cable robust (able to withstand axial or radial loading) and capable of surviving in environments that would otherwise damage the optical fiber and render the cable inoperable.
Current technology has focused on placing an optical fiber in a buffer tube such that the optical fiber is parallel to the buffer tube and therefore unbent. Such a parallel unbent configuration for the optical fiber provides the low attenuation characteristics mentioned above. Buffer tubes have been made from steel and are referred to in the art as “K” tubes. Additionally, buffer tubes made from plastics are known in the art. Current technology, whether employing steel or plastic buffer tubes, is focused on eliminating excess fiber length from the resulting buffer tube so that low attenuation results when optical signals travel in the optical fiber.
As mentioned above, optical fibers are made from glass. To ensure long life of the optical fiber, the stress experienced by the optical fiber during its working life should not exceed 25 percent of the proof test stress. Commercial grade optical fiber has a proof test stress of 100,000 pounds per square inch (psi) (equivalent to an elongation of approximately one percent) and high strength optical fiber has a proof test stress of 300,000 psi (equivalent to an elongation of approximately three percent). Thus, the maximum working elongation that an optical fiber can sustain spans the range of 0.25 to 0.75 percent. An optical fiber is subjected to additional elongation due to the elongation properties of a strength member incorporated into a cable.
Typical cables used to contain optical fibers and buffer tubes experience a non-recoverable increase in length referred to as “constructional slack” or “constructional elongation,” which is typically removed during the first load cycle applied to the cable. The constructional elongation ranges from 0.2 to 0.6 percent for typical cable designs. Existing optical fiber/buffer tubes, incorporated into a cable that elongates during the first load cycle, experience a non-recoverable elongation that further reduces the elongation that the optical fiber can safely experience during its working life. As an example, an optical fiber that has a safe working elongation of 0.25 percent experiences a reduction of 0.2 percent due to the nonrecoverable release of constructional slack, resulting in only a 0.05 percent safe working elongation being available for working loads applied to the cable.
A variation on this example can occur if the cable experiences a nonrecoverable release of constructional slack of 0.6 percent. The elongation experienced by the optical fiber during the application of a load that produces a 0.25 percent strain in the cable will produce a 0.85 percent strain in the optical fiber. Such a state of strain will drastically shorten the life of the cable; this presents a problem.
Constrained by this problem, cables are over designed by necessity (from the perspective of the strength member) in order to keep the elongation experienced by the optical fiber to within a safe working limit (elongation resulting from application of 25 percent of proof test stress). Over designs of 10 to one or 20 to one are not uncommon in the telecommunications industry; this can present a problem.
FIG. 1 illustrates a stress strain curve for an existing cable generally at 100. For example, in the cable design illustrated in FIG. 1, the optical fiber can undergo a maximum working elongation of 0.1 percent (due to constructional elongation placing the optical fiber in a state of permanent prestress) and the cable can safely undergo a maximum working elongation of two percent (from a consideration of strength member limits). Referring to FIG. 1, stress is plotted on the vertical axis at 104 and strain is plotted on the horizontal axis at 106, the resulting stress-strain curve is indicted at 102. The maximum safe working stress (as governed by the strength members) is indicated at 110 and is equivalent to a load of 20,000 pounds. The strain corresponding to the maximum working stress is indicated at 112 and is equal to two percent, in this example, where ΔL is a change in length of a gauge length L. Cable yield is indicated at 108. Stresses and strains indicated on the figures, including FIG. 1 are not to scale, these values are plotted for illustrative purposes only; therefore, absolute values should not be inferred from any of the figures.
The constrained maximum operating stress that the optical fiber can safely withstand is indicated at 116 (equivalent to an applied load of 1,000 pounds) and the corresponding constrained maximum strain is indicated at 114, 0.1 percent, in this example. Inspection of the stress/strain curve 102, between 114 and 112, indicates that a majority of the mechanical strength available in this cable design cannot be realized in practice due to the constraint imposed by the existing configuration of the optical fiber contained within.
Existing metal K tubes provide a further limitation to the applied load and plastically deform when the applied load produces a strain of 0.6 percent in the K tube. Regardless of the material used for the buffer tube, metal or plastic, current constructions of buffer tubes and optical fibers necessitate the use of a substantial amount of strength member in a cable design such that the resulting cable must be operated at only a fraction of its maximum working load (as governed by the strength member) in order to prevent damage to the optical fiber. In conjunction with the existing optical fiber/buffer tube, strength members have been made out of metallic elements such as steel.
In other applications of cables incorporating optical fibers, such as neutrally buoyant cables or small diameter cables, such mechanical over design and use of metallic strength members are not feasible. Synthetic strength members can be utilized to reduce the total cable weight. Synthetic strength members loaded to 50 percent of their breaking strength can elongate approximately from 1.2 to 1.7 percent during the application of a working load. Added to the working load elongation is the constructional elongation of 0.2 to 0.6 percent resulting in a total elongation range that an optical fiber must endure of 1.4 to 2.3 percent in order to fully utilize the full load carrying capacity of a cable. Such an elongation range places the working elongation of the optical fiber above the safe limit described above; this presents a problem.
The cables described above, incorporating optical fibers, can be used in interferrometric optical systems, wherein small differences in path length (synonymous with small changes in optical phase) are important. Such interferrometric systems are sensitive to changes in path length that equate to strain on the order of 1×10−13 In such systems, it has been found that existing cable designs are sensitive to mechanical excitations, such as stress, vibration, etc.; this presents a problem.
Cables that incorporate optical fibers are also used as optical fiber hookup cables, where an optical connection is made between a first point and second point. Such hookup cables must be flexible, sustain loads applied in both the axial and radial directions, and in some cases be insensitive to environmentally born mechanical excitations such as vibration, stress, etc. Existing cable constructions present problems to this type of use for the reasons discussed above.